The present invention relates to the production of lasing microdevices and specifically to self-assembled lasing microcavities consisting of highly luminescent nanocrystals.
Colloidal semiconductor nanocrystals have been exploited in electro-optical devices like light emitting diodes,1 lasers2,3 and solar cells. This trend is driven by the advanced synthesis of nanocrystals of various materials, sizes and shapes and will benefit from progress in their organization into ordered architectures by self-assembly. So far, lasing was difficult to achieve in nanocrystals because optical gain competed with Auger recombination above threshold, which shortened excited state lifetime to tens of picoseconds,4 and laser feedback was possible only via external resonators.
The achievement of optical gain in colloidal semiconductor nanocrystals is challenging because two main obstacles have to be faced, namely trapping of excited carriers at surface/interface defect states5 and Auger recombination. Trapping can be reduced effectively by growing a passivating shell of a higher band gap material around the emitting dot.5 Auger recombination takes place in multiexciton regime and is highly efficient, with time constants ranging from a few picoseconds to a few hundreds of picoseconds.4 One solution is to reduce the Auger process by working with semiconductor nanocrystals in which at least one spatial dimension is not in the strong confinement regime. Colloidal quantum rods (QRs) with a few nm in diameter offer the advantage of retaining the strong quantum confinement along two spatial directions, while their absorption cross section is significantly increased with respect to that of spherical QDs having similar optical features. As a result they show much longer gain lifetime than QDs6 (up to nearly 100 ps), which can be increased further if the rods are coated with an epitaxial shell of suitable composition and thickness (150 ps being the longest gain lifetime reported so far7). Core-shell QRs thus represents the more promising solution towards lasing, provided that suitable ways to construct optical resonators become available.
So far, solely examples of lasers based on semiconductor nanocrystals have been reported,2,8 in which an external resonator was adopted for generating the optical feedback (either a physical cavity or a feedback mechanism such as scattering in random lasing). In all these reported cases the QRs behaved therefore like dyes in conventional lasers. The need for an external resonator imposed several difficulties in matching the fabrication steps of wet chemical synthesis and deposition with the processes needed to fabricate the resonator.
US 2007/0178615 describes semiconductor nanocrystals-based optical devices and a method of preparing such devices. The optical device (e.g. the laser cavity) is formed by nanocrystal films on a surface which may be planar or not. The film is created by processing a nanorod solution with electromagnetic radiation, such as laser radiation or by coating techniques.